1. Field of the Invention
This invention relates to recording and reproducing system for a color video signal, and more particularly to an improved chroma signal recording and reproducing system which minimizes deviation of hue from the original hue as well as deviation of color saturation from the original color saturation and eliminates drop-out or missing of color information thereby ensuring a high quality of reproduced pictures.
2. DESCRIPTION OF THE PRIOR ART
Various types of chroma signal recording and reproducing systems have been proposed hitherto. Among them, two types, that is, the low-band conversion type and the color-difference line-sequential FM type are typical of those commonly employed in magnetic picture recording and reproducing apparatus for home use, such as video tape recorders. The basic principle, merits and demerits of these two types will now be described before describing the present invention in detail.
(1) Low-band conversion type
According to this type, a subcarrier is subjected to quadrature two-phase balanced modulation by two color difference signals to obtain a modulated chrominance signal commonly used in the NTSC system or PAL system, the frequency band of the chrominance signal being then converted into that lower than the FM band of a luminance signal, and this low-band converted chrominance signal is superposed on the FM luminance signal to be recorded together with the luminance signal on a magnetic recording medium such as a video tape. In the playback mode, the low-band converted chrominance signal is separated from the reproduced signal to be then restored to the original high-band chrominance signal by a frequency converter. The restored chrominance signal is then synthesized with the separately demodulated luminance signal to provide the color video signal commonly used in the standard NTSC system or standard PAL system.
Therefore, the low-band conversion type exhibits the following merits among others:
(a-1) By virtue of the quadrature two-phase balanced modulation, the two color difference signals can be recorded in multiplex in the same frequency band without widening the frequency band occupied by the chrominance signal.
(a-2) In an apparatus such as a VTR in which a color video signal is recorded by moving a magnetic recording medium relative to a magnetic head, there occurs a time base variation in the reproduced signal when the speed of the magnetic recording medium moving relative to the magnetic head varies. However, because of the fact that this time base variation causes merely a phase variation of the chrominance signal, the conversion of the frequency band of the chrominance signal into its original high-frequency band alleviates the adverse effect of the time base variation when the chrominance signal is recorded in the form converted into the low-frequency band.
(a-3) The simple procedure for merely converting the frequency band of the chrominance signal is only required for the recording and reproduction of a color TV signal used in the NTSC system or PAL system.
(a-4) The chrominance signal is provided by modulation according to an AM mode. Therefore, no beat interference occurs even when adjoining two tracks may be scanned by the magnetic head at the same time in the playback mode, and crosstalk can be easily eliminated by means of phase shift (PS) or phase inversion (PI). Thus, the low-band conversion type is suitable for guardband-less recording.
On the other hand, the low-band conversion type has the following demerits among others:
(b-1) The chrominance signal is provided by modulation according to the so-called AM mode. Therefore, when a level variation occurs in the reproduced signal due to, for example, a variation of the reproducing engagement between the magnetic recording medium and the magnetic head, the level variation cannot be removed resulting in deviation of color saturation from the original color saturation.
(b-2) Since the chrominance signal obtained by the balanced modulation does not include the carrier component, a continuous reference carrier having the same frequency and phase as those of the subcarrier used during the modulation must be prepared and used for the purpose of synchromous detection during demodulation of the color signal. However, when a time base variation is present in the reproduced chrominance signal, the reference carrier cannot completely follow up the time base variation resulting in deviation of hue from the original hue, even when, for example, an APC circuit is incorporated to produce the reference carrier on the basis of the color burst signal.
(b-3) The low-band converted color signal is superposed on the FM luminance signal to be recorded together with the luminance signal on a magnetic recording medium. Therefore, when a distortion of third order typical of magnetic recording appears in the recording and reproducing system, a beat interference (moire) due to cross modulation appears in the reproduced luminance signal. In order to alleviate this moire interference, it is necessary that the frequency of the subcarrier converted into the low band is to be selected at a special valve to fully utilize the effect of interleaving or that the level of the low-band converted color signal relative to the FM luminance signal is to be severely restricted.
In regard to the level variation described in (b-1), incorporation of a conventional AGC circuit which controls the gain on the basis of the detected burst level has been effective for compensation of the sensitivity difference between the magnetic heads and for correction of the mean level in each field. However, it has been difficult for the AGC circuit to suppress an instantaneous level variation.
(2) Color-difference line-sequential FM type:
According to this type, the two color difference signals are subjected to frequency modulation (FM), and the FM color difference signals are alternately recorded at a time interval of one horizontal scanning period (1H). In the playback mode, in lieu of the color difference signal omitted in the record mode, the signal corresponding to the preceding one horizontal scanning period (1H) is obtained by delaying the reproduced signal by 1H, and such a signal is used to supplement the omitted signal.
Therefore, this color-difference line-sequential FM type exhibits the following merits among others:
(c-1) By virtue of the use of frequency modulation (FM), a level variation that may occur can be removed by a limiter, and any deviation of color saturation from the original color saturation does not occur.
(c-2) By virtue of the use of the frequency modulation, deviation of hue from the original hue does not also occur even in the presence of a time base variation.
On the other hand, the color-difference line-sequential FM type has the following demerits among others:
(d-1) According to its basic principle, the color signal information drops out at an interval of one horixontal scanning line. Therefore, the vertical resolution of color is degraded.
(d-2) The azimuth effect is not so markedly exhibited since the frequency band used for recording the FM color signal is generally set to be lower than the FM band of the luminance signal, and, even when an inclined azimuth arrangement is employed, simultaneous scanning for reproduction of information from adjoining two tracks by the magnetic head results in occurrence of a beat interference. Therefore, the color-difference line-sequential FM type is not suitable for guardband-less recording, and it is difficult to increase the picture recording density.